1. Field of the Invention
The present invention relates to the field of modulating immune responses by administering an agent that influences the structure of sialyl galactosides that are present on cells involved in immune responses.
2. Background
While all eukaryotic cell surfaces are covered by oligosaccharides, vertebrate cells specifically display a more diverse and complex repertoire in part by the production of unique asparagine-linked oligosaccharide structures (Kornfeld and Kornfeld, Ann. Rev. Biochem., 54:631 (1985); Varki and Freeze, In: Subcellular Biochemistry: Membrane Biogenesis, (eds, Maddy and Harris, Plenum Press, New York), v.22, pp.71 (1994)). The vertebrate glycosyltransferase gene superfamily controls the biosynthesis and diversification of cell surface oligosaccharides (Schachter, Curr. Opin. Struct. Biol., 1, (1991); Kleene and Berger, Biochim. Biophys. Acta, 1154:283 (1993); Marth, Glycoconjugate J. 11, 3 (1994)). These Type II transmembrane enzymes harbor exquisite substrate specificities and are organized in the Golgi apparatus to function in the stepwise production of oligosaccharides. Oligosaccharide diversification in phylogeny, ontogeny, cell activation and tumorigenesis is due to organism- and cell type-specific glycosyltransferase gene expression profiles (Varki and J. D. Marth, in: Seminars in Developmental Biology 6:127 (1995)).
Intercellular recognition and adhesion is a complex phenomenon responsible for numerous cellular interactions, such as fertilization, cell migration, organ formation, and immune defense. The high selectivity required by these processes is often provided by lectins, a class of nonimmunogenic proteins that bind carbohydrates selectively and noncovalently. Typically, lectins recognize and bind carbohydrates associated with proteins and lipids on the cell surface of the apposing cell. For instance, the CD22 lectin a transmembrane glycoprotein found exclusively on B lymphocytes and is known to play a role in the immunologic activation of these cells (Campana et al. (1985) J. Immunol. 134: 1524-1530; Stamenkovic and Seed (1990) Nature 345: 74-77; Dorken et al. (1989) in Leucocyte Typing IV: White Cell Differentiation Antigens, eds. Knapp et al. (Oxford Univ. Press, Oxford), pp. 63-64). CD 22 has been found associated with the antigen receptor and is a target for tyrosine kinase phosphorylation on the cytoplasmic domain, which thereby recruits various signal transduction molecules (Schulte et al. (1992) Science 258: 1001-1004; Leprince et al. (1993) Proc. Nat""l. Acad Sci. USA 90: 3236-3240). The extracellular domain of CD22 specifically binds the Sia6LacNAc trisaccharide (Powell et al. (1993) J. Biol. Chem. 268: 7019-7027; Sgroi et al. (1993) J. Biol. Chem. 268: 7011-7018; Powell and Varki (1994) J Biol. Chem. 269: 10628-10636). This trisaccharide ligand exists on several lymphoid cells. Lymphocyte interactions involving CD22 binding to CD45 have been reported (Stamenkovic et al. (1991) Cell 66: 1133-1144). As CD22 itself carries Sia6LacNAc, homotypic binding interactions have been shown to occur and may play a regulatory role in immune function (Braesch-Anderson and Stamenkovic (1994) J. Biol. Chem. 269: 11783-11786; Hanasaki et al. (1995) J. Biol. Chem. 270: 7533-7542). These results suggest that CD22 and Sia6LacNAc are a letin-ligand pair with the potential to control immune cell surface interactions. However, a relatively simple model for CD22 function has not developed from analyses of CD22 null mice by several laboratories (O""Keefe et al. (1996) Science 274: 798-801; Otipoby et al. (1996) Nature 384: 634-637; Sato et al. (1996) Immunity 5: 551-562; Nitschke et al. (1977) Curr. Biol. 7: 133-143). Results obtained have inferred both positive and negative roles for CD22 in B lymphocyte immune function, suggesting that CD22 may modulate threshold signaling responses from the antigen-receptor complex.
Despite progress in the identification of lectins and their carbohydrate ligands, a need exists for the discovery of new compositions and methods for the control of cellular interactions associated with them. Such compositions and methods can be used to modulate a number of processes, such as immune responses. The present invention addresses these and other needs.
The invention provides methods of inhibiting an immune response in a mammal by administering to the mammal a therapeutically effective amount of an agent which causes a reduction in amounts of a sialylated oligosaccharide present in the mammal. The sialylated oligosaccharides generally have a formula that, at the non-reducing end, terminates with Siaxcex12-6Gal- or Siaxcex12,3Gal-. In preferred embodiments, the sialylated oligosaccharide includes Siaxcex12-6Galxcex21-4GlcNAc or Siaxcex12,3Galxcex21-3GalNAc. To inhibit a B lymphocyte-mediated immune response, an agent is used that reduces the amount of oligosaccharides having Siaxcex12-6Gal-, and T lymphocyte-mediated immune responses are inhibited by administering an agent that reduces the amount of oligosaccharides having Siaxcex12,3Gal-.
In another embodiment, the invention provides methods of detecting immunodeficiency or other immune system disorders in a mammal. These methods involve contacting a sample from the mammal, which samples typically contain lymphoid cells, with a detection agent which specifically binds to a sialylated oligosaccharide. The sialylated oligosaccharides of interest generally include the formula Siaxcex12-6Gal- or Siaxcex12,3Gal-. A substantial absence of binding of the detection agent to the lymphocyte is indicative of immunodeficiency.
The invention also provides methods of detecting immunodeficiency or other immune system disorders in a mammal, by detecting the presence or absence of ST6Gal or ST3Gal I sialyltransferase activity in a sample obtained from the mammal. A substantial absence of ST6Gal or ST3Gal I sialyltransferase activity is indicative of immunodeficiency.
Another embodiment of the invention provides mammalian cells that have a genome which includes an inactivating mutation in a gene that encodes a sialyltransferase. The mutation can be one or more of a deletion, a nonsense mutation, an insertion, and a missense mutation. The inactivating mutation can be present in a coding region or a regulatory region of a gene that, in nonmutated form, encodes a sialyltransferase.
Also provided are chimeric non-human mammals that have cells in which the genomes contain an inactivating mutation in a gene that encodes a sialyltransferase. Included are chimeric mammals in which fewer than all cells have the inactivated gene, as well as transgenic mammals in which all cells include the inactivated gene.